An Unlikely Hero

Around the world, scientists are racing to develop treatments and vaccines to fight Covid-19. But any new medications they discover can’t be given to the public until pharmaceutical companies test each and every batch. They need to ensure the products haven’t been contaminated with harmful bacteria that could make people sick. And for that, they rely on help from an odd-looking, prehistoric creature: the horseshoe crab.

Around the world, scientists are racing to develop treatments and vaccines for Covid-19. When new medications are found, they’ll be given to the public. But first, pharmaceutical companies must test each and every batch. They need to make sure the products haven’t been contaminated with harmful bacteria that could make people sick. To do the testing, they need help from an odd-looking, prehistoric creature. It’s the horseshoe crab.

Although the Covid-19 pandemic is new, the horseshoe crab’s role in health care isn’t. For decades, a substance in the animal’s blood has been used to make sure that certain medications and medical devices are germ-free. Scientists use this substance to test anything that will come in contact with a patient’s bloodstream. That way, whenever a doctor gives you a shot, for instance, they know that the needle and medicine it delivers are clean and safe.

Each year, labs collect roughly 500,000 horseshoe crabs from the ocean and draw their blood so it can be used in these tests. This practice saves many human lives. Unfortunately, tens of thousands of crabs die each year in the process. The animals face other threats too, like habitat loss and pollution, that further jeopardize their numbers. To keep both horseshoe crab populations and people healthy, scientists are turning to an alternative to replace the animals’ blood that’s so vital to the biomedical industry.

The Covid-19 pandemic is new, but horseshoe crabs’ role in health care isn’t. For decades, researchers have relied on a substance in the animal’s blood. This substance helps make sure that certain medications and medical devices are germ-free. Scientists use it to test anything that will enter or touch a patient’s bloodstream. For example, it’s used to test needles and the medicine they deliver. So when doctors give you a shot, they know everything is free of bacteria.

Each year, labs collect about 500,000 horseshoe crabs from the ocean. They draw their blood for use in these tests. This practice saves many human lives. Sadly, tens of thousands of these crabs die each year. The animals face other threats too. For example, habitat loss and pollution jeopardize their numbers. But their blood is vital to the biomedical industry. Scientists want to keep both horseshoe crab populations and people healthy. So they’re turning to a replacement for the animals’ blood.

The Atlantic horseshoe crab (Limulus polyphemus) lives along the eastern seaboard of the United States (see Horseshoe Crab Ranges). Three other species live in coastal areas of Asia. Despite their name, the animals aren’t actually crabs. They’re more closely related to spiders and scorpions. Scientists estimate that horseshoe crabs have been around for about 400 million years—they appeared more than 150 million years before the dinosaurs.

Humans and most vertebrates have red blood. But other animals have green, purple, or—if they’re like horseshoe crabs—blue blood. The blue color comes from the element copper. In 1956, a researcher noticed something else about horseshoe crab blood: It changes from a liquid to a gel when it comes into contact with gram-negative bacteria. Many of the bacteria that belong to this group can make people ill. The liquid-to-gel reaction that occurs in the horseshoe crab’s blood is called clotting. It’s how the animal’s disease-fighting immune system responds to injuries. If bacteria from seawater gets into a crab’s body through a cut, the crab’s blood clots, creating a gel that seals the wound.

The Atlantic horseshoe crab (Limulus polyphemus) lives along the eastern coast of the United States (see Horseshoe Crab Ranges). Three other species live in coastal areas of Asia. But the animals aren’t really crabs. They’re more closely related to spiders and scorpions. Scientists think that horseshoe crabs have been around for about 400 million years. They existed more than 150 million years before the first dinosaurs.

Humans and most vertebrates have red blood, but other animals have green or purple blood. And some, like horseshoe crabs, have blue blood. The blue color comes from the element copper. In 1956, a researcher noticed something else about horseshoe crab blood. It changes from a liquid to a gel when it touches gram-negative bacteria. Many of the bacteria in this group can make people ill. The liquid-to-gel reaction in the horseshoe crab’s blood is called clotting. It’s how the animal’s disease-fighting immune system responds to injuries. Say, for example, bacteria from seawater enters a crab’s body through a cut. The crab’s blood clots in response. It creates a gel to seal the wound.

Researchers figured out how to collect the clotting agent, a substance called Limulus amebocyte lysate (LAL), from horseshoe crabs’ blood. Then they devised ways to use the substance to identify bacterial contamination in medical products. Laboratory technicians mix LAL with samples of injectable medicines. And they wash medical devices, like needles and syringes, with extremely pure water and then use LAL to test the water for bacteria. A clotting reaction signals contamination; no reaction means the samples are safe.

The clotting agent is a substance called Limulus amoebocyte lysate (LAL). Researchers found a way to collect the substance from horseshoe crabs’ blood. Then they devised ways to use it to detect harmful bacteria in medical products. Laboratory workers mix LAL with samples of injectable medicines. And they wash needles, syringes, and other medical devices with extremely pure water. They use LAL to test the water for bacteria. A clotting reaction means harmful bacteria is present. No reaction means the samples don’t contain bacteria.

Each year, fishers harvest thousands of horseshoe crabs and send them to labs. There, technicians bend the horseshoe crabs at the hinge between the front and back sections of their carapace, or shell, and insert a needle into the exposed flesh (see Anatomy of a Horseshoe Crab). They drain a third of the animal’s blood into a bottle and then spin the blood in a centrifuge. The machine separates out blood cells called amebocytes. Workers add water to these cells, which soak up the liquid until they burst and release LAL. The clotting agent is freeze-dried and sent to pharmaceutical companies to test their products.

Once labs collect horseshoe crabs’ blood, fishers immediately return the animals to the ocean. However, anywhere from 5 percent to 30 percent of the crabs die after their release. Those that do survive face other threats. Hundreds of thousands of horseshoe crabs are caught each year for use as fishing bait. In Asia, the crabs are also harvested for their blood and then sold for their meat and shells instead of being returned to the ocean. Pollution, coastal development, and climate change are destroying the horseshoe crabs’ ocean habitat too. That’s not just bad news for crabs—it hurts other animals as well. Red knots and other birds feed on the eggs that female horseshoe crabs lay on sandy beaches each May and June. The birds rely on the eggs as a vital source of energy for their annual migration.

Each year, fishers catch thousands of horseshoe crabs and send them to labs. Workers in the labs bend the horseshoe crabs at the hinge between the front and back parts of their carapace, or shell. Then they insert a needle into the exposed flesh (see Anatomy of a Horseshoe Crab). They drain a third of the animal’s blood into a bottle, and they spin the blood in a centrifuge. The machine separates out blood cells called amoebocytes. Workers add water to these cells. The amoebocytes soak up the liquid until they burst. That releases LAL. The clotting agent is freeze-dried. Then it’s sent to pharmaceutical companies to test their products.

After labs collect horseshoe crabs’ blood, fishers quickly return the animals to the ocean. But 5 percent to 30 percent of the crabs die after that. The ones that survive face other threats. Hundreds of thousands of horseshoe crabs are caught each year for use as fishing bait. In Asia, the crabs are also collected for their blood. But they’re not returned to the ocean. Instead, they’re sold for their meat and shells. Pollution, coastal development, and climate change are destroying the horseshoe crabs’ ocean habitat too. That’s not just bad news for crabs. It also hurts other animals. Female horseshoe crabs lay eggs on sandy beaches each May and June. Red knots and other birds feed on these eggs. The birds need this energy source for their yearly migration.

The disruptive practice of harvesting horseshoe crabs for their blood has motivated scientists to develop a synthetic, or lab-made, alternative to LAL. Researchers first created the substance, called recombinant factor C (rFC), in 1997 using copies of a gene found in horseshoe crab blood. This unit of hereditary information carries the instructions for producing one of the proteins—large biological molecules—responsible for horseshoe crabs’ clotting reaction. The scientists inserted the gene into microbes that produce rFC as a substitute for LAL.

The practice of collecting horseshoe crabs for their blood is disruptive. That’s why scientists have developed a substitute for LAL. It’s a synthetic substance, made in a lab. In 1997, researchers created recombinant factor C (rFC). They used copies of a gene in horseshoe crab blood. This unit of hereditary information carries the instructions for producing a protein, a large biological molecule. It’s one of the proteins that causes horseshoe crabs’ clotting reaction. The scientists inserted the gene into microbes, and the microbes produced rFC.

Switching to rFC won’t solve all of horseshoe crabs’ problems. “But it can help reduce our impact on the animals,” says Jay Bolden, a biologist at the pharmaceutical company Eli Lilly headquartered in Indiana. And it will ensure important medical testing can continue, helping people too. With a surge in testing of new products to fight Covid-19, some people are concerned that there won’t be enough horseshoe crab blood to go around. But the companies that sell LAL say the supply is up to the demand. Many pharmaceutical manufacturers aren’t ready to abandon the tried-and-true method of testing with LAL, either. It’s backed by decades of research and still considered the gold standard for detecting bacteria that could seriously harm people. But other companies believe a growing amount of data shows rFC is an effective alternative. Eli Lilly has already begun using the synthetic testing agent to check the purity of several medications, including one it’s developing to treat Covid-19.

One thing is clear: Horseshoe crabs are vital in battling the pandemic. Someday, people might not need to collect horseshoe crab blood at all. But no matter what, the crabs’ contribution to health care will live on—and hopefully, so will these amazing creatures.

Using rFC won’t solve all of horseshoe crabs’ problems. “But it can help reduce our impact on the animals,” says Jay Bolden, a biologist at the pharmaceutical company Eli Lilly in Indiana. And it will make sure important medical testing can continue. That will help people too. All of the new products to fight Covid-19 must be tested. Some people worry that there won’t be enough horseshoe crab blood to go around. But the companies that sell LAL say there is plenty. Many pharmaceutical companies aren’t ready to stop using LAL, either. This tried-and-true testing method is backed by decades of research. It’s still considered the gold standard for detecting harmful bacteria. Other companies believe a growing amount of data shows rFC is a good substitute. Eli Lilly has already begun using rFC to test several medications. One is a medication it’s developing to treat Covid-19.

One thing is clear. Horseshoe crabs are vital in fighting the pandemic. Someday, people might not need to collect horseshoe crab blood at all. But the crabs’ contribution to health care will live on. And hopefully, these amazing creatures will too.